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Origin of Life: Not From Soup, But Gas

The first glint of sunlight at dawn reflects off a lake on Titan, taken by NASA's Cassini space probe 17 December 2009.

Credit: NASA

BRISBANE: Organic molecules, including amino acids and nucleotides of DNA, may form in some planets' atmospheres, such as Titan's, Saturn's largest moon, and even ancient Earth's, astronomers said.

"Our results show that it is possible to make very complex molecules in the outer parts of an atmosphere," said Sarah Hörst, who presented the work at the annual meeting of the Division for Planetary Science of the American Astronomical Society.

Hörst and her colleagues mimicked Titan's atmosphere, and found biological molecules such as nucleotides and amino acids, the building blocks of DNA and proteins, could be made.

DNA, proteins may comes from atmosphere

Titan's hazy atmosphere is made up of methane and other small organic compounds, said Hörst, a planetary scientist at the University of Arizona, in Tucson.

Those aerosols are constantly being bombarded with ultraviolet radiation and charged particles coming from the sun, which could provide the energy necessary to break up the simple compounds and rearrange them into prebiotic molecules like nucleotides.

Hörst's team created a Titan-like atmosphere in a stainless steel reaction chamber and bombarding it with microwaves. The researchers identified about 5,000 molecular forms, including all five nucleotides used to make DNA and RNA and more than half of the amino acids used to make proteins.

Just like other planets and ancient Earth?

"The interesting part for us is that we now know you can make pretty much anything you want in an atmosphere," said Hörst. "Who knows this kind of chemistry isn't happening on planets outside our solar system?"

Titan's atmosphere is very different to Earth's atmosphere today, but researchers think it is an analog for Earth's atmosphere before oxygen-producing life forms evolved, more than 2.3 billions years ago.

It is thought that first ingredients of life on Earth were formed in a primodial soup, but the new results suggest that it might have happened in the atmosphere instead, Hörst said.

Testing Titan, the real thing

The researchers studied information from NASA's Cassini space probe, which flies through the edge of Titan's atmosphere regularly and analyses it with an mass spectrometer. While the probe detected both small and large molecules, its instruments aren't capable of identifying the larger, potentially pre-biotic molecules, Hörst said.

George Cody, a geochemist at the Carnegie Institution for Science, Washington DC, US, said that the results should provide impetus for a return to Titan with more sophisticated instruments.

"Titan is an extremely interesting 'planetoid' that may well inform us about what was possible in earliest history of our planet," Cody said.